Determining UT1 at U.S. Naval Observatory by Analysis of Global Positioning System Orbits

Peter Kammeyer

Abstract:	Determination of polar motion by analysis of Global Positioning System (GPS) orbits became fully operational at the U.S. Naval Observatory in 1995. Polar motion determination uses the GIPSY-OASIS II orbit determination program developed at the Jet Propulsion Laboratory. In the process of determining polar motion, accurate orbits for GPS satellites and approximate values of UT1 R, the Earth rotation parameter UT1 with tidal terms of periods thirty-five days and less removed, are determined. This paper concerns an ongoing effort to improve the approximate UT1 R by more detailed analysis of GPS orbit planes. Each week from September 1995 to February 1996, several satellites were chosen for which orbit-plane motion, modelled using observations made many months earlier, appeared adequate to estimate UT1 R. A UT1 R value obtained at U.S. Naval Observatory from analysis of VLBI and other data was used to adjust the value of UT1 R at the beginning of the previous two GPS weeks, and values of UT1 R later in that period were determined by comparing predicted and observed changes in ascending-node position. This gave UT1 R values with a root-mean- square (rms) error of 70 microseconds. After software improvements, estimation of UT1 resumed in April 1996. A UT1 value was set at the beginning of April, and GPS analysis alone was used to construct a continuous series of UT1 estimates for later times. Each week, three to four GPS satellites were used to estimate UT1 . In April and May 1996, UT1 estimates had an rms error of 40 microseconds, but in June 1996 error in the estimated UT1 began increasing at a rate of 200 microseconds per week. UT1 is now estimated by analyzing over a dozen GPS orbits but using no detailed modelling of orbit- plane motion. The rms error is 80 microseconds for a two-week interval between calibrations. Further improvements in accuracy appear to depend on more detailed modelling of orbit plane motion.
Published in:	Proceedings of the 9th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GPS 1996) September 17 - 20, 1996 Kansas City, MO
Pages:	1283 - 1290
Cite this article:	Kammeyer, Peter, "Determining UT1 at U.S. Naval Observatory by Analysis of Global Positioning System Orbits," Proceedings of the 9th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GPS 1996), Kansas City, MO, September 1996, pp. 1283-1290.
Full Paper:	ION Members/Non-Members: 1 Download Credit Sign In